As is known in the art, a light source or lamp generally refers to an electrically powered man made element which produces light having a predetermined color such as white or near white. Light sources may be provided, for example, as incandescent light sources, fluorescent light sources and high-intensity discharge (HID) light sources such as mercury vapor, metal halide, high-pressure sodium and low-pressure sodium light sources.
As is also known, fluorescent and HID light sources are driven by a ballast. A ballast is a device which by means of inductance, capacitance or resistance, singly or in combination, limits a current provided to a light source such as a fluorescent or a HID light source. The ballast provides an amount of current required for proper lamp operation. Also, in some applications, the ballast may provide a required starting voltage and current. In the case of so-called rapid start lamps, the ballast heats a cathode of the lamp prior to providing a strike voltage to the lamp.
As is also known, one type of ballast is a so-called magnetic or inductive ballast. A magnetic ballast refers to any ballast which includes a magnetic element such as a laminated, iron core or an inductor. One problem with magnetic ballasts, however, is that the relatively low frequency drive signal which they provide results in a relatively inefficient lighting system. Furthermore, magnetic ballasts tend to incur substantial heat losses which further lowers the efficiency of lighting systems utilizing a low frequency magnetic ballast.
In an attempt to overcome the low efficiency problem caused by the low frequency operating characteristic of magnetic ballasts as well as the inability to operate in an instant-start mode, attempts have been made to replace magnetic ballasts with electronic ballasts. Electronic ballasts energize the lamps with relatively high frequency drive signals and can provide strike voltages which allow instant-start lamp operation. One problem with known electronic ballasts, however, is that they utilize a relatively large number of circuit components which reduces reliability and maintainability of the electronic ballast while increasing cost.
Electronic ballasts generally include a rectifier circuit for converting an alternating current (AC) input signal to a direct current signal (DC) and an inverter circuit to drive the load with an AC signal. The inverter circuit can be a circuit having resonant inductive, capacitive and resistive elements coupled in various parallel and/or series configurations to provide resonant operation of the circuit. Inverter circuits generally include switching elements arranged in a half or full bridge configuration with the switching elements controlled in various ways. For example, U.S. Pat. No. 5,220,247 discloses a conventional half bridge inverter circuit configuration having the switching elements controlled by inductors inductively coupled to the resonant inductive element. Conduction of the switching elements can be controlled with a pulse width modulation circuit as disclosed in U.S. Pat. No. 4,415,839. U.S. Pat. No. 5,434,477 discloses a half bridge inverter circuit coupled to a boost circuit having a switching element in common with the inverter circuit.
One problem associated with known half and full bridge inverter circuits is that during cross conduction, the switching elements effectively short circuit positive and negative rails of a power supply. The power supply can include a rectifier circuit. It will be appreciated that circuit components can be severely damaged in a short amount of time in the presence of such a short circuit. Even if cross conduction of multiple switching elements is prevented from a circuit operation standpoint, transients, electromagnetic interference (EMI) pulses, and other such events can result in cross conduction of switching elements. Furthermore, cross conduction prevention and/or protection schemes require additional circuit components thereby adding cost and increasing space requirements.
Another disadvantage associated with known electronic and magnetic ballast circuits is the output isolation transformer typically used to meet safety requirements. In particular, the load current must be limited, i.e., 43 mA at present, in the event that one end of a lamp is removed from the circuit to protect an operator from severe electrical shock. The isolation transformer is bulky and presents a significant cost in the manufacturing of a ballast.
It would, therefore, be desirable to provide a circuit for driving a load which affords circuit protection during cross conduction of switching elements. It would further be desirable to provide a circuit that affords current limiting protection to a user without an output isolation transformer.